The present invention relates to an air-gap-insulated exhaust manifold of an internal combustion engine, in particular in a motor vehicle, having the features of the preamble of claim 1.
Such an exhaust manifold is known from DE 44 760 A1, for example, and includes a collecting line extending in a longitudinal direction, an outlet opening oriented in the longitudinal direction, multiple inlet openings oriented across the longitudinal direction and a flange that contains the inlet openings and extends in the longitudinal direction. Such an exhaust manifold is formed by an outside pipe mounted on the flange and by an inside pipe inserted into the outside pipe, forming an air-gap insulation. A design which saves on materials is achieved with the known exhaust manifold by the fact that a gas-carrying outside pipe section which is provided in the area of at least one of the inlet openings leads from the respective inlet opening to a respective inside pipe inlet at a distance from the flange.
This design takes into account the fact that the areas of the exhaust manifold in direct proximity to the inlet opening are under less thermal stress than the collecting line in which there is an increased mass flow of hot exhaust gases and a detour in the exhaust gases. Omitting the inside pipe in the area of the inlet openings permits a considerable savings of material.
Air-gap-insulated double-wall exhaust manifolds are being used increasingly in the exhaust systems of internal combustion engines, in particular in motor vehicles, where they ensure optimum operation of a downstream catalyst. First, they cause a reduced dissipation of the heat of the exhaust gas to the environment so that the exhaust can be sent to the respective catalyst at a higher temperature. This is important in particular during the warm-up phase of the internal combustion engine because the catalyst may then reach its operating temperature very rapidly. Secondly, the air-gap-insulated exhaust manifolds reduce the heat acting upon components, arranged adjacent to the exhaust manifold in the engine space, for example.
In the case of exhaust manifolds of this type, the air-gap insulation necessarily results in the inside pipe being exposed to higher temperatures than the outside pipe. Consequently, the inside pipe expands more than the outside pipe during operation of the internal combustion engine. The resulting problems lead more complex designs with the known traditional exhaust manifolds. For example, the inside pipe may be designed in multiple parts, in which case the individual inside pipe parts are mounted so they are movable with respect to one another via sliding seats. In this way the individual inside pipe parts are able to move toward one another to compensate for thermal expansion. However, the manufacturing cost associated with this is comparatively high.